1. Field of the Invention
This invention relates to novel rubber compositions or compounds and more particularly, to silicone rubber compounds having good resistance to hot water and good thermal conductivity and processability. The invention also relates to blends of the silicone rubber compounds with other types of rubbers by which the blends have good storage stability and releasability from other organic rubbers before vulcanization and rubber articles obtained therefrom.
2. Description of the Prior Art
In general, silicone rubbers have the feature of good heat resistance and weatherability, but are disadvantageous in that they are inferior in breaking strength, resistance to water and hot water as compared to ordinary organic hydrocarbon rubbers.
Extensive studies have been heretofore made in order to overcome the above disadvantages of silicone rubbers.
For instance, for the improvement of physical strength, attempts have been made to use a reinforcing silica filler having a relatively large surface area of, for example, over 300 m.sup.2 /g or to use, as a polyorganosiloxane, a combination of a polyorganosiloxane having vinyl groups in small amounts and a polyorganosiloxane having vinyl groups in large amounts.
In addition, there has been also proposed substitution of part of substituents of organopolysiloxane green rubbers with phenyl groups (in which, in almost all the cases, other substituents are substantially composed of methyl groups, with the proportion of the phenyl group being not larger than 5 mole % of the total substituents).
In order to improve the water and hot water resistance, there is known a method in which reinforcing silica is treated with organosilicon compounds such as polydiorganosiloxanes, organosilanes or organosilazanes on the surfaces thereof. This method is based on the concept that the reason why silicone rubber compounds have poor water and hot water resistances is considered due to the fact that water is adsorbed on the surface of the reinforcing silica filler and the reinforcing effect of silica on polyorganosiloxane is impeded. To avoid this, the silica is covered with hydrophobic groups on the surfaces thereof whereby the water adsorption is prevented.
However, the above methods have the respective drawbacks: although the use of silica fillers having relatively large surface areas and the method using a combination of polyorganosiloxanes having a small amount and a large amount of vinyl bonds may achieve the improvement of physical strength to an extent, they have the drawbacks in poor water and hot water resistances.
The method of introducing phenyl groups into polyorganosiloxanes is also disadvantageous in that since silica is used as the reinforcing agent, so that although the physical strength of the resulting silicone rubber is slightly improved, the elasticity and elongation lower with poor water and hot water resistances.
The method for treating the silica surfaces with organosilicon compounds is very difficult in completely removing the moisture from the silica surfaces. If the moisture is completely removed, the reinforcement is not ensured, so that there cannot be obtained satisfactory effects of remarkably improving the water and hot water resistances of silicone rubber.
On the other hand, it may occur that carbon black is used as a reinforcing agent. However, with known polymers mainly composed of carbon black-added polydimethylsiloxane, typical of which is used as a conductive silicone rubber composition, little affinity between carbon black and the polymers is expected, so that carbon black cannot be dispersed sufficiently and the kneading operation is very difficult. Thus, it takes a very long time for the kneading. In addition, carbon black separates during the course of processing with the vital disadvantage that carbon black is locally concentrated on the surfaces thereof.
As described above, silicone rubbers have good heat resistance, weatherability and releasability, but are inferior in breaking strength, water and hot water resistances to ordinary organic rubbers. Accordingly, extensive studies were heretofore made on rubber compounds or compositions obtained by blending both types of rubbers in order to obtain rubber compounds which have both features of silicone rubber compounds and organic rubbers.
Hitherto known rubber blends of organic rubbers and silicone rubbers include mere blend or mixture of both types of rubbers (as is disclosed, for example, in Japanese Laid-open Patent Application Nos. 54-41957, 55-139604 and 56-76444), and rubber compounds in which polyorganosiloxanes are bonded with C.dbd.C double bonds of organic rubbers (as disclosed, for example, in Japanese Laid-open Patent Application Nos. 54-157149, 55-15497, 55-31817, 56-76440 and 56-76441).
However, because of the absence of affinity of polyorganosiloxanes for organic rubbers, organic rubbers and polyorganosiloxanes are very liable to separate. Therefore, mere blends are disadvantageous in that they are extremely poor in storage stability under unvulcanized conditions, giving an adverse influence on the quality and characteristics of final product and thus the supervision of the blends is very difficult.
The rubber compounds obtained by bonding polyorganosiloxanes to C.dbd.C double bonds of organic rubbers are not satisfactory with regard to the releasability from organic rubbers.
All the known methods or rubber compounds which use reinforcing silica fillers have the vital drawback that the reinforcing ability of silica deteriorates by the action of water or hot water, which is attributed to the moisture adsorption of silica.
As will be appreciated from the foregoing, the known rubber compounds obtained by mixing organic rubbers and polyorganosiloxanes have the features of the respective types of rubbers to an extent, but have the respective serious drawbacks and have not been unsatisfactory for practical applications. Accordingly, there is a demand of rubber compounds which overcome the above drawbacks and have high utility.